Abstract:

A method of manufacturing powder metal plates comprising feeding a
predetermined mass of metal powder onto a moving tape (101), restricting
the metal powder by surrounding the metal powder with vibrating boundary
walls (201, 202) extending parallel to the direction of movement of the
tape, rolling the metal powder at an ambient temperature to form a green
compact strip (GS), continuously sintering the green compact strip in a
furnace (400), forming the green compact strip to a net shape part (NS)
while in the furnace, and cooling the net shape part in a non-oxidizing
environment (404) at a temperature in excess of 1000 degrees Celsius.

Claims:

1. A method of manufacturing powder metal plates comprising:feeding a
predetermined mass of metal powder onto a moving tape (101);restricting
the metal powder by surrounding the metal powder with vibrating boundary
walls (201, 202) extending parallel to the direction of movement of the
tape;rolling the metal powder at an ambient temperature to form a green
compact strip (GS);continuously sintering the green compact strip in a
furnace (400);forming the green compact strip to a net shape part (NS)
while in the furnace; andcooling the net shape part in a non-oxidizing
environment (404) at a temperature in excess of 1000 degrees Celsius.

2. The method as in claim 1 further comprising exposing the net shape part
to an oxidizing atmosphere at a temperature at or below 1000 degrees
Celsius.

3. The method as in claim 1 further comprising continuously sintering the
green compact strip at a temperature of approximately 1400 degrees
Celsius.

4. The method as in claim 1 further comprising rolling the green compact
strip to a net shape part occurs at a temperature of approximately 1400
degrees Celsius.

5. The method as in claim 1, wherein the furnace comprises an induction
furnace.

8. The method as in claim 6 further comprising the step of cutting the
green compact strip before hot forging.

9. The method as in claim 2, wherein cooling the net shape part in an
oxidizing environment is for a period in the range of approximately ten
to twelve hours.

10. A method of manufacturing powder metal plates comprising:feeding a
predetermined mass of metal powder onto a moving tape;restricting the
metal powder by surrounding the metal powder with vibrating boundary
walls extending in the direction of movement of the tape;rolling the
metal powder at an ambient temperature to form a green compact
strip;continuously sintering the green compact strip in a furnace;forming
the green compact strip to a net shape part while in the furnace;
andcooling the net shape part in an oxidizing environment at a
temperature below 1000 degrees Celsius.

11. The method as in claim 10 further comprising continuously sintering
the green compact strip at a temperature of approximately 1400 degrees
Celsius.

12. The method as in claim 10, wherein rolling the green compact strip to
a net shape part occurs at a temperature of approximately 1400 degrees
Celsius.

13. The method as in claim 10, wherein the furnace comprises an induction
furnace.

16. The method as in claim 14 further comprising the step of cutting the
green compact strip before hot forging.

17. A method of manufacturing powder metal plates comprising:feeding a
predetermined mass of metal powder onto a moving tape;restricting the
metal powder by surrounding the metal powder with vibrating boundary
walls extending in the direction of movement of the tape;rolling the
metal powder at an ambient temperature to form a green compact
strip;continuously sintering the green compact strip on a conveyer in a
furnace;rolling the green compact strip to a net shape part while in the
furnace at the sintering temperature; andcooling the net shape part in a
non-oxidizing environment at a temperature in excess of 1000 degrees
Celsius.

18. The method as in claim 17, wherein continuously sintering the green
compact strip at a temperature of approximately 1400 degrees Celsius.

19. The method as in claim 17, wherein rolling the green compact strip to
a net shape part occurs at a temperature of approximately 1400 degrees
Celsius.

Description:

FIELD OF THE INVENTION

[0001]The invention relates to a method of manufacturing powder metal
plates comprising feeding a predetermined mass of metal powder onto a
moving tape, restricting the metal powder by surrounding the metal powder
with vibrating boundary walls extending in the direction of movement of
the tape, rolling the metal powder at an ambient temperature to form a
green compact strip, continuously sintering the green compact strip on a
conveyer in a furnace, forming the green compact strip to a net shape
part while in the furnace, and cooling the net shape part in a
non-oxidizing environment at a temperature in excess of 1000 degrees
Celsius.

BACKGROUND OF THE INVENTION

[0002]The existing art for manufacturing certain powder metal plates,
including fuel cell plates, is to use a powder comprising 95% Cr and 5%
Fe. The powder is compacted in a press to the desired shape. The green
compact is sintered in a furnace at 1120 degrees Celsius. The sintered
part is then forge/coin (re-strike) in a press to increase the density
and then finally re-sintered at 1400 degrees Celsius.

[0003]Representative of the art is U.S. Pat. No. 6,436,580 (2002) which
discloses a method of manufacturing porous sheet metal sheet comprising
metal powders are spread on a feeding belt or a supporting sheet which is
continuously fed; the feeding belt or the supporting sheet on which the
metal powders have been spread is passed through a sintering oven; and
the metal powders are sintered, with adjacent uncompressed metal powders
in contact with each other partly and gaps present therebetween.
Consequently, contact portions of the metal powders are integrated with
each other and the gaps are formed as fine pores.

[0004]What is needed is a method of manufacturing powder metal plates
comprising feeding a predetermined mass of metal powder onto a moving
tape, restricting the metal powder by surrounding the metal powder with
vibrating boundary walls extending in the direction of movement of the
tape, rolling the metal powder at an ambient temperature to form a green
compact strip, continuously sintering the green compact strip on a
conveyer in a furnace, forming the green compact strip to a net shape
part while in the furnace, and cooling the net shape part in a
non-oxidizing environment at a temperature in excess of 1000 degrees
Celsius. The present invention meets this need.

SUMMARY OF THE INVENTION

[0005]The primary aspect of the invention is a method of manufacturing
powder metal plates comprising feeding a predetermined mass of metal
powder onto a moving tape, restricting the metal powder by surrounding
the metal powder with vibrating boundary walls extending in the direction
of movement of the tape, rolling the metal powder at an ambient
temperature to form a green compact strip, continuously sintering the
green compact strip on a conveyer in a furnace, forming the green compact
strip to a net shape part while in the furnace, and cooling the net shape
part in a non-oxidizing environment at a temperature in excess of 1000
degrees Celsius.

[0006]Other aspects of the invention will be pointed out or made obvious
by the following description of the invention and the accompanying
drawings.

[0007]The invention comprises a method of manufacturing powder metal
plates comprising feeding a predetermined mass of metal powder onto a
moving tape, restricting the metal powder by surrounding the metal powder
with vibrating boundary walls extending parallel to the direction of
movement of the tape, rolling the metal powder at an ambient temperature
to form a green compact strip, continuously sintering the green compact
strip in a furnace, forming the green compact strip to a net shape part
while in the furnace, and cooling the net shape part in a non-oxidizing
environment at a temperature in excess of 1000 degrees Celsius.

BRIEF DESCRIPTION OF THE DRAWINGS

[0008]The accompanying drawings, which are incorporated in and form a part
of the specification, illustrate preferred embodiments of the present
invention, and together with a description, serve to explain the
principles of the invention.

[0017]Fuel cells are one of the most promising power generation systems
for the future. Typically, a solid oxide fuel cell (SOFC) consists of the
fuel electrode (anode) and the oxygen electrode (cathode) which are
interconnected by an ion-conducting electrolyte. The electrodes are
electrically coupled to an electric load by conductors (wires) outside
the cell. Solid oxide fuel cells can be operated in a temperature range
of approximately 800° C. to 1000° C. in hydrogen with 5% or
50% water at current densities ranging from 0.25 A cm-2 to 1 A
cm-2. The fuel cell typically uses hydrogen as the fuel. Accordingly
the powder metal fuel cell plates must be fabricated with this design
condition in mind.

[0018]FIG. 1 is a table describing prior art fuel cell technologies. As it
can be seen in FIG. 1, each cell has certain material requirements. As
per FIG. 1, one of the fuel cell types is the solid oxide fuel cell
(SOFC). There are six common alloys for the interconnect material of SOFC
type of fuel cells. The chromium base type may be composed of 95%
chromium, 5% iron, with or without yittria.

[0019]This invention improves the manufacturing process of the chromium
base SOFC fuel cell plates, also known as interconnect plates, or any
other material that cannot be made into sheet metal and coined
conventionally due to its lack of formability or ductility. Due to the
properties expected from the fuel cell plates the chromium base type is
preferred by some fuel cell manufacturers.

[0020]The inventive process comprises powder feeding, tape casting, and
powder rolling. The combination of all three processes, plus the use of
vibrating walls at the feeder is an inventive way to efficiently and
quickly make uniform density and uniform thickness fuel cell plates.

[0021]Unlike other possible materials, such as different grades of
stainless steel that can be made in sheet form and subsequently coined
(stamped) in a press, the 95% Cr-5% Fe cannot be made into sheet metal
form and cannot be coined due to its very poor formability properties.
Consequently, the existing art for manufacturing these plates is to mix
95% Cr with 5% Fe powder; separately compact the powder in a powder metal
press to the desired shape; sinter the parts in a furnace at 1120 degrees
Celsius; forge/coin (re-strike) the part in a press to increase the
density and re-sinter at 1400 degrees Celsius. Each discrete step
involves repeatedly handling and moving each part.

[0022]The inventive process replaces the prior art manufacturing process
with the disclosed process which is simpler, more efficient, and less
expensive. Further, it requires less capital in equipment and can be
performed in one continuous manufacturing cell. Further, forming the
plates at high temperature removes substantially all of porosity in the
finished part.

[0023]FIG. 2 is a side view schematic of the inventive process. The
process uses a powder comprising a mix of 95% Cr with 5% Fe. The powder
is compacted by forming the powder into a green compact strip of material
similar to a sheet metal using powder feeding, tape casting, and powder
rolling. The process generates a green compact strip having a uniform
density and uniform thickness with the desired width.

[0024]Process.

[0025]First, generally in region (1) powder feeding 100 is utilized to
feed the metal powder uniformly into a die cavity with a sweeping moving
head 101. The powder is typically stored in a bin 102 which feeds by
gravity to head 101. Head 101 is feed by a vibratory feeder 104.

[0026]In the inventive process, metal powder feeding is done on a moving
plastic (or other suitable material) tape 200 where the metal powder is
restricted by side walls (201, 202). Side walls 201, 202 are parallel to
the feed direction. Powder feeder 101 continuously discharges metal
powder thereby keeping the strip always filled with a uniform amount of
metal powder. Tape 200 drive system is synchronized and assisted by
conveyor 204 which engages the underside of tape 200. Tape casting is
typically used to feed ceramic powders into a curing furnace. At each end
of the strip, the beginning (input feed) side is restricted with a wall
203, and at the other end by the two rollers 301, 302, see FIG. 2.

[0027]The side walls give the metal powder feeding system the height
needed to sweep-in feeding the metal powder on the tape. Walls (201, 202)
also prevent the loose powder from falling sideways. A wall 204 also
controls the powder as it flows onto the tape.

[0028]In order for walls (201, 202, 204) not to create too much friction
or stick to the metal powder, they are being vibrated at all times. Walls
(201, 202) end at the rollers 300, but the tape is pulled into and
through the rollers, namely, by the roller 302.

[0029]FIG. 3 is a top view schematic of the inventive process. Next,
powder rolling is performed. Although rollers 301, 302 are rolling a flat
green strip, one has a top-flat hill 302 and the opposite one has a
bottom-flat valley 303, see FIG. 4A. The flat width (D) is the width of
the plastic tape 200 which is the width of the green compact strip (GS)
as it emerges from the rollers 301, 302. Plastic tape 200 is rolled away
from the green strip after rolling.

[0030]Next, generally in region (2) the continuous green compact strip
(GS) is fed into a furnace 400 supported by a moving high temperature
conveyer 402. Green compact strip GS travels through a gate 401 that is
used on powder metal sintering furnaces to contain an oxygen free furnace
environment. The travel length for the strip GS is long enough to bring
it up to the 1400 degree Celsius (or any other desired) temperature and
deliver it to the net-shape rollers 501, 502 which are inside the furnace
400. Hot rolling of the strip GS occurs at the sintering temperature.
Dwell time in the furnace at the sintering temperature is approximately
30 minutes.

[0031]The thickness of the green strip GS in the first rolling operation
300 is calculated and adjusted to deliver the exact weight (mass) of
material (plus a fraction of approximately one percent for safety, if
needed) at a constant width to the final rolling operation at rollers
500. The density change is accounted for in going from the cold green
compact strip to the hot net-shape part. As a result, proper control of
the mass flow from feeder 101 is important. Control of the volume of the
metal in the strip GS is less critical as the volume and thickness will
be reduced by hot rolling at 500, which reduces or completely eliminates
porosities.

[0032]The net-shape rollers 500 apply the same principles of hills 503 and
valleys 504, see FIG. 4B, to assure side compaction is accurate and to
eliminate side flash, which represents waste. Each surface 503 and 504
comprises the surface features suitable to impress the proper form and
features in the finished net shape part NS. In terms of side confinement,
this is similar to a rolling mill operation for most metals where the
sides are kept constant.

[0033]The rolling operation in the furnace requires that rollers (501,
502) be internally water cooled with inserts in the forming areas. Since
the rolling is done at high temperatures in the furnace the compaction is
to net final shape with little or no porosity left in the finished part.
In order for the final net shape hot rolling to be successful, the green
compacted strip GS must be of uniform density and uniform thickness.

[0034]After the rolling operation 500 is finished, the net shape parts
(NS) are cooled in the non-oxidizing environment 404 of the furnace and
subsequently they exit the furnace 600. Once again, a protective reducing
atmosphere is needed to prevent oxidation of the material, generally in
region (3).

[0035]After exiting the furnace, small amounts of flash (if any) between
parts are de-flashed by any of known processes. There should be no flash
on the sides due to side restrictions in the rollers 300 and 500. The
rolling arrangement can be either two rolls, or two rolls supported by
larger back supporting rolls. This is similar to rolling mills, where by
using smaller rolls a more concentrated force over a smaller surface area
is achieved, while the larger rolls prevent the deflection of smaller
diameter rolls.

[0036]Since forming by rolling presses the powder in a much localized and
narrow band comparing with coining in a press, it is possible to generate
much higher localized compressive stresses on the powder than a press
compaction, where the entire part is subjected to the forming stresses at
once. Consequently, the inventive process achieves a high compaction,
especially at high temperatures so that porosity is completely
eliminated.

[0037]Using the prior art, powder metal presses for manufacturing such
fuel cell plates can be very costly due to the high tonnage requirements.
However, using the inventive process localized compacting by powder
rolling requires simpler, lower tonnage, and lower cost equipment.

[0038]FIG. 6 is a side view of the rollers. Each roller (302, 502)
comprises an outer surface 303, 503 respectively. Each surface 303, 503
is the "negative" or inverse of the plate being rolled. Each surface 303,
503 cooperates with surface 304, 504 respectively, see FIG. 4A and 4B. In
an alternative embodiment, roller 302 does not have a "negative" feature
surface, instead the surface is simply flat as shown in FIG. 4A.

[0039]FIG. 7 is a perspective view of the system. The net shape parts NS
exit the furnace in a continuous strip or as individual parts separated
after the final rolling. The parts are then separated and processed for
storage or installation. Roller conveyor 405 supports the green strip GS.
Roller conveyor 406 supports the net shape parts NS.

[0040]The inventive process can be modified in many different ways. A
significant part of the process is the first step of generating a strip
of green compacted powder with consistent and uniform thickness and
consistent and uniform density, similar to that for a sheet metal. The
disclosed process teaches a conventional furnace and hot rolling for the
second stage.

[0041]Other alternatives include induction heating rather than using a
conventional furnace. A protective non-oxidizing atmosphere is needed.
The induction heating eliminates the need to use a long furnace.

[0042]Another alternative is hot forging rather than hot rolling. The
green compacted strip can be fed into a forging press after being heated
in the furnace or induction heating. A press type knife can be used to
cut a certain length of the hot strip that is then fed into the forging
press. This results in the delivery of a precise and uniform weight of
the metal into the die cavity. In the alternate embodiment the forging
press is located in the same location as rollers 500. Although hot
rolling is preferred due the need to apply localized pressure, forging at
the high temperature of approximately 1400 degrees Celsius (or any other
desired temperature) can also significantly reduce the porosities.

[0043]In yet another alternative, during the cooling stage the net formed
parts may be introduced to an oxidizing atmosphere upon reaching a
temperature of approximately 1000 degrees Celsius and below. This allows
the part to be stabilized for its intended service conditions in fuel
cell service. For example, this can be accomplished by having the parts
exit into a belt furnace zone which has a controlled temperature below
1000 degrees Celsius with an appropriate atmospheric air flow. FIG. 5 is
a graph showing a cooling trend for the part wherein the part exits the
reducing atmosphere and it cooled and then enters a holding zone for an
oxidizing atmosphere. The hold time in the oxidizing atmosphere is on the
range of approximately ten to twelve hours.

[0044]The weight of metal powder delivered to the cavity could be a
fraction of approximately one percent more than the weight of the
finished part to assure complete filling of the die cavity. The extra
weight will result in a very slight and thin layer of flash that can be
removed easily.

[0045]The advantages of the inventive process include a process with much
lower cost; a process with much lower capital cost; easier processing
steps and eliminating complexity; improved quality, namely, the porosity
in the part is going to be zero or extremely low. Further it enables use
of only one continuous and compact manufacturing cell to make the
finished part.

[0046]Although the inventive process can be used to manufacture solid
oxide types of fuel cells, this method may be used in any similar
application where there is a need for plates that can handle high
temperature electrolyte without corroding and with a given degree of
expansion at higher temperatures. Lastly, this process can be used for
any material that cannot be made into sheet metal and has little or no
formability.

[0047]Although a form of the invention has been described herein, it will
be obvious to those skilled in the art that variations may be made in the
construction and relation of parts without departing from the spirit and
scope of the invention described herein.